Reply to ASK-AN-EARTH-SCIENTIST

Subject: Earthquakes on the New Madrid Fault

8th grade students (Sarah and Amanda) studying the New Madrid Fault
Zone asked the following 9 questions

1. Is there any known explanation for the strange animal behavior
that occurs around earthquakes
Earthquakes have all sorts of precursors, some of which animals (and
humans, for that matter) might be sensitive to. The precursors include
cracking of rock as it is strained, changes in magnetic field, release
of radon gas from cracked rock, ground water variations, changes in the
color of well water, etc. It is clear that some animals (horses, geese,
fish) are quite sensitive to small foreshocks and low-level cracking.
The trouble with animal reactions is that they are not
reliable--sometimes animals may show signs of concern and nothing
happens, sometimes animals sense nothing before an earthquake. This
reflects on the nonlinear and random nature of earthquakes themselves.

2. Is there any explanation for the "frost that lacks the chill"?
I don't understand the question because I don't know anything about
weather conditions in the winter of 1811-1812. My guess is that whether
there was a chilling frost or not had nothing to do with the
earthquakes.

3. Is there any known explanation for the earthquake light?
We don't know the explanation yet, but there are several suggestions.
It might be related to piezoelectricity (voltage induced by squeezing
rock) or triboluminescence (photon discharge on breaking atomic bonds).
To get an idea what earthquake lights are like here is a simple
triboluminescence experiment for you: get some wintergreen LifeSavers,
take them into a dark room, and smash them with a hammer. You'll see a
green flash with each hammer blow.

4. Is it possible for the Mississippi to change course again with
another earthquake?
Sure, that's how Reelfoot Lake was formed.

5. How did the formation of the Reel Foot Lake change the path of
the Mississippi?
The motion in the earthquakes of 1811-1812 apparently was complex with
mostly strike-slip (horizontal motion) along NE-SW faults, but with
both reverse faulting and normal faulting south of the town of New
Madrid. Because the faults crossed the path of the Mississippi,
vertical displacement of the ground dammed the river and diverted it.
I'm not sure if Reelfoot Lake is the old bed of the Mississippi or if
it is a remnant of the swamp formed when the Mississippi was dammed.

6. Is it possible for the earthquake to b as strong or stronger than
the quakes in 1811 & 1812? Since this is overdue, will that impact on
the strength of the quake?
The center of the North American continent is accumulating strain
because of east-west compression. The earthquakes of 1811-1812 were the
release of this strain. The strain accumulation, however, is very slow.
The average time from one 1811-sized event to the next is probably much
more than 500 years. I would not expect any earthquakes to be larger
than 1811-1812, and I would not expect such earthquakes to occur for
another couple of hundred years. Seismologists at the University of
Memphis figure that there is only a 3-4% chance of a magnitude 8
earthquake along the New Madrid seismic zone within the next 50 years.
In other words a big earthquake is NOT overdue. Smaller earthquakes are
a different matter, however, so people should know what to do in an
earthquake.

7. Was the weather affected by the earthquakes?
No. Since the Mississippi was dammed it is probable that in the
immediate vicinity of the new swamp things were more humid, but that
would have been a very local effect.

8. Is it possible for dangerous waves to occur on the Mississippi?
If an earthquake disrupted the bed of the Mississippi, as it did in
1812, there would be dangerous waves generated by the sudden damming of
the river and the change in the slope of its bed. These would only be
dangerous very close to the fault. They would very rapidly loose
height as they moved around the bends of the river. In 1812 the maximum
wave height was apparently about fifteen feet--enough to drive boats a
quarter mile up tributary creeks.

9. Is there an area wherre the quake is most likely to occur?
The New Madrid seismic zone extends at least 150 miles from Cairo,
Illinois to Marked Tree, Arkansas. The entire thing ruptured in
1811-1812. The next big earthquake could happen anywhere along this
trace.

If you want to learn more about New Madrid, try the public information
page at the Center for Earthquake
Research and Information at Univ. Memphis. They have a feedback page so
if you have any questions they would probably help you. Another good
source of information is the
Virtual Times. They
have some good links, but some of what appears there is a bit too sensational.